The artificial capillary culture technique provides a pericellular microenvironment that closely resembles the in vivo state. This may allow normal cells to maintain their differentiated functions for prolonged periods of time in vitro. Cells are maintained in a physiologic state without many of the artifactual conditions imposed by more standard methods of tissue culture. The technique is, therefore, especially well suited for the culture of cells of endocrine origin. Responses of the cells to various physiologic agents, as well as the secretion of various hormones over long periods, are being studied, with the possible application to continuous production of hormones for laboratory and clinical uses. Preliminary experiments have shown that follicles form within rat ovarian tissue grown within the extracapillary space of the culture units. These cultures are being used to study the effects of PRL, GH, FSH, and estrogen upon the synthesis and secretion of prostaglandin by ovarian tissue.